Definition.

Tincture is the terms used to describe heraldic colouring.
Tincture can be either a metal or a colour or a fur. White and Yellow are referred to as Metals because of their similarity to silver and gold while other "colours" are referred to as colours. Furs refer to some patterns that are thought to resemble ermine or squirrel fur in various colours.

Rule of tincture.

The general rule is that colour should not appear immediately on another colour, nor a metal on another metal. Furs may be placed on metals or colours but not other furs.